(1) Field of the Invention
The present invention relates to a highly efficient and adaptive microelectronic fabrication process for adjusting color filter formation to optimize pixel color sensitivity balance and to control the red:green:blue gain-ratio in optoelectronic semiconductor array imaging devices.
(2) Description of Prior Art
Synthetic reconstruction of color images in solid-state analog or digital video cameras is conventionally performed through a combination of an array of optical microlens and spectral filter structures and integrated circuit amplifier automatic gain control operations following a prescribed sequence of calibrations in an algorithm.
Typically solid-state color cameras are comprised of charge-coupled device (CCD), Charge-Injection Device (CID), or Complementary Metal-Oxide Semiconductor (CMOS) structures with planar arrays of microlenses and primary color filters mutually aligned to an area array of photodiodes patterned onto a semiconductor substrate. The principal challenge in the design of solid-state color camera devices is the trade-off between adding complexity and steps to the microelectronic fabrication process wherein color filters are integrally formed in the semiconductor cross-sectional structure versus adding complexity and integrated electronic circuitry for conversion of the optical analog signals into digital form and signal processing with color-specific automated gain-control amplifiers requiring gain-ratio balance. The trade-off between microelectronic fabrication process complexity versus electronic complexity is determined by a plurality of factors, including product manufacturing cost and optoelectronic performance.
The problem in semiconductor array devices for color imaging fundamentally arises because different color pixels in the matrix exhibit varying spectral sensitivity to the different wavelengths or frequencies contained in the incident image light. For example, a photodiode element or sensor in the matrix array that is more sensitive to red light than blue light creates an imbalance in the captured image.
The imbalance can be corrected either by precompensation in the color-filter array, achieved by designing the fabrication process adaptively, or, digitally after the analog-to-digital conversion (ADC) step. It is well known, however, that post-processing signals after ADC is too late to avoid adding undesired quantization noise. Electrical signal precompensation to correct the color sensitivity imbalance, analogous to the aforementioned alternative color-filter process approach, is possible, and, is performed before ADC by adding to each color-pixel output signal a signal quantity derived from a color-specific gain controlled amplifier and additional circuitry for the control of the red:green:blue gain-ratio.
Typical CMOS image sensors incorporate more functional integration than do CCD sensors, which are manufactured by a specialized process, thereby making it a challenge to add image-processing circuitry to the chips. In contrast, CMOS sensors are made with the same high-volume processes used to build most computer chips, so digital circuitry can be added to enhance sensor functionality. The CMOS sensor chips typically integrate pixel array, timing logic, sampling circuits, amplifiers, reference voltage supplies, and ADC""s. The CCD sensor chips require a minimum of two support chips to accomplish the same functions as CMOS. The increased integration offered by CMOS sensors can reduce system complexity and allow smaller camera designs. But, more circuits on CMOS chips increase the potential for noise from one section of the chip to interfere with the operations of another section.
A frequent problem is the noise generated by the digital section can interfere with the highly sensitive front-end analog circuits and degrade image quality. In either case of CCD or CMOS devices, it is clearly seen that the preferred method for improving color balance in semiconductor array imaging devices is the analog optical filter, with attendant simplicity of circuitry and avoidance of the electrical noise interference introduced by added circuitry needed to execute color-compensation functions.
Color-photosensitive integrated circuits require carefully configured color filters to be deposited on the upper layers of a semiconductor device in order to accurately translate a visual image into its color components. Conventional configurations may generate a color pixel by employing four adjacent pixels on an image sensor. Each of the four pixels is covered by a different color filter selected from the group of red, blue and two green pixels, thereby exposing each monochromatic pixel to only one of the three basic colors. Simple algorithms are subsequently applied to merge the inputs from the three monochromatic pixels to form one full color pixel. The color filter deposition process and its relationship to the microlens array formation process determine the production cycle-time, test-time, yield, and ultimate manufacturing cost. It is an object of the present invention to teach color-filter processes which optimize these stated factors.
In addition to the dynamic range and noise of the individual photodetectors, the resolution or fidelity of a CCD or CMOS image is influenced by the overall array size, individual pixel size, spacing, and fill factor. Quantitatively, this performance is described by a wavelength-dependent modulation transfer function (MTF) that relates the two-dimensional Fourier transform of the input image to that of the output. In addition to the obvious effects of pixel aperture and shape, the MTF of an array is affected by spatial carrier diffusion, temporal diffusion, and optical diffraction.
While color image formation may be accomplished by recording appropriately filtered images using three separate arrays, such systems tend to be large and costly. Cameras in which a full color image is generated by a single detector array offer significant improvements in size and cost but have inferior spatial resolution. Single-chip color arrays typically use color filters that are aligned with individual columns of photodetector elements to generate a color video signal. In a typical stripe configuration, green filters are used on every other column with the intermediate columns alternatively selected for red or blue recording. To generate a color video signal using an array of this type, intensity information from the green columns is interpolated to produce green data at the red and blue locations. This information is then used to calculate a red-minus-green signal from red-filtered columns and a blue-minus-green signal from the blue ones.
Complete red-minus-green and blue-minus-green images are subsequently interpolated from this data yielding three complete images. Commercial camcorders use a process similar to this to generate a color image but typically utilize more complicated mosaic-filter designs. The use of alternate columns to yield color information decreases the spatial resolution in the final image.
FIG. 1 exhibits the conventional Prior Art vertical semiconductor cross-sectional profile and optical configuration for color image formation. Microlens 1 residing on a planarization layer 2 which serves as a spacer collects a bundle of light rays from the image presented to the video camera and converges the light into focal cone 3 onto photodiode 8 after passing through color filter(s) 4 residing on planarization layer 5, passivation layer 6, and metallization layer 7.
FIG. 2 illustrates a representative Prior Art example for the generation of a color image by a single photodetector array 9 by using a color filter mask comprised of green stripe 10, red stripe 11, green stripe 10, blue stripe 12, green stripe 10, and red stripe 11. In this scheme, green filters are placed over alternate photodetector columns. Red and blue filters alternate in the spaces between them. Interpolation routines are used to generate three-color data for all pixel positions.
FIG. 3 provides a schematic cross-sectional view for a three-dimensional version of the scheme shown in FIG. 2. In both FIG. 2 and FIG. 3, the microlens arrays are intentionally left out to simplify the discussion of the filter fabrication sequence and configuration for color pixel synthetic reconstruction. In FIG. 3, photoactive regions 13 on semiconductor substrate 14, are successively overlayed with a patterned conductor layer 15, blanket passivation layer 16, blanket planarizing layer 17, first color filter layer 18 for green, second color filter layer 19 for red, and third color filter layer 20 for blue. Reading from left to right in FIG. 3, one observes the order of green, red, green, blue, and, periodic repetitions of this sequence.
Park et al in U.S. Pat. No. 5,877,040 shows a CCD with a convex microlens formed integrally on the planarization layer above a photodiode element of a CCD array such that the focal-distance of the lens may be positioned by adjustment of the intervening film-layer thicknesses. Following a dry-etch step of the substrate, the set of convex microlenses are formed in a second planarization layer above a first planarization layer containing color filters. No specification of the physical or spectrophotometric properties of the color filters, nor any specification of particular sequences for multiple color filter layers is provided.
Pace et al in U.S. Pat. No. 5,143,855 describe a color filter process in which the primary objects of the invention are the fabrication method for microregistration of dyeable polymer layers aligned with the photosensing elements and the provision for making contact openings for a bonding pad pattern in an inorganic passivation layer on which the color filters are formed. Here again, no specification of the order of the color filter sequence nor spectrophotometric properties are provided.
Hawkins et al in U.S. Pat. No. 5,677,202 teach a color filter process in which the principal object of the invention is the achievement of all color filter elements of the array to have entirely coplanar top and bottom surfaces with no overlap and with minimal gaps or no gaps between adjacent filter elements. In common with Prior Art, no specification of the spectrophotometric properties of the color filters nor the order of the sequence of the colors is provided.
In U.S. Pat. No. 5,358,810, Yoshino et al describe a low-cost, mass manufacturing color filter process for forming a stripe color filter in various resin-based materials for a liquid crystal display device in an electrode substrate. The liquid crystal application is significantly different from the solid-state color image camera, including illumination sources and conditions, no need for microlens and related alignments, no semiconductor photosensors and required mutual alignments to microlens and filter sequences, and, no need to balance the red:green:blue gain-ratios required for pixel creation algorithms.
Suginoya et al in U.S. Pat. No. 5,770,449 teach forming color filters for a multicolor liquid crystal display by electrodeposition and by patterning using color filters as masks. Applicable to forming and patterning a plurality of electrodes for liquid crystal displays, this method would not be practical for the precision of photolithographic alignments of microlenses and color filters in semi-conductor matrix array color imaging devices.
The fabrication process for formation of a stepped transmittance gradation mask for providing neutral-density attenuators for color filters used in liquid crystal display devices is described in U.S. Pat. No. 5,725,975 by Nakamura et al. Photolithographically patterned semi-transparent films, illustrated with chromium compounds, are interposed with color filters to adjust transmittances to predetermined values for each of the plurality of prescribed areas of the color filter gradation mask. Transmittance values for the color filters comprising the gradation mask are adjusted by calculated thickness variations of the semitransparent metal film areas.
The color filter processes and structures taught in the present invention can be clearly distinguished from the Prior Art and will be shown to include fewer process steps and improved accuracy and precision. A principal object of the present invention is to teach a particular sequential order for the formation and vertical profile of the color filters integrated into semiconductor array imaging cameras and related devices.
In accord with a principal object of the present invention, there is provided by the present invention a manufacturing method and microelectronic fabrication process sequence which minimizes the number and task-times of the operational steps required in the production of semiconductor arrays for color imaging devices. In current conventional color filter processes for CCD/CMOS image sensor devices, the most ordinary coating sequence is in the order of green (G) followed by red (R) followed by blue (B), designated G/R/B. However, the relative thicknesses of G/R/B directly effects the color gain-ratio. The spectral absorption of each of the G/R/B filters to specific wavelengths of light commonly employed in photoresist exposure, such as the I-line, are each different. If the thickness of the blue coating is to too high, the absorption to the I-line will be excessive and result in color pixel sensitivity imbalance. Since the integrated circuits (IC) responsible for pixel color gain are relatively fixed in dynamic range by design, the present invention solves the problems of avoiding complicated circuit redesign, time-consuming and expensive photolithographic mask-making and/or deposition system retooling by providing a real-time spectrophotometric and algorithmic feedback process control method suitable for the color pixel thickness adjustments in a specified coating technique and particular layer-order to result in a wider, robust process window. By changing the color coating sequence to blue first, followed by red, followed by green, designated B/R/G, a first layer with high transmittance, defined as greater than or equal to 80%, widens the process window. Similarly, the coating arrangement B/G/R provides the color pixel structure (G,B) or (G,R) with wider process window, reduced blue layer thickness than green first, and, stronger adhesion to the substrate to avoid pixel lift-off.
A principal object of the present invention is, therefore, to disclose the method and means to regulate the relative thickness of each color filter layer to obtain the desired gain balance without changes to any circuit layout.
It is another object of the present invention to directly vary the dye or pigment concentration loading of each color filter based on spectrophotometric measurement data to provide analog precompensation adjustments to control and optimize the pixel color sensitivity balance and red:green:blue gain-ratio of the integrated circuit amplifiers in optoelectronic array imaging devices. Adjustments in dye or pigment concentration loading are made, as in the previous thickness variable case, during the color filter fabrication process.
Common to all the objects of the present invention is high spectral efficiency through the elimination of the need for additional steps for layers of neutral-density attenuators.
Another object of the present invention is to teach a highly efficient and flexible microelectronic fabrication process for integrated color filter formation which is inherently compatible with integrated microlens array formation and the precision mutual alignments to each other and to the photodiode elements of the matrix array of a color video camera or other semiconductor array color imaging devices.
A still further object of the present invention is to provide a fabrication method for color filter and microlens array formation for high reproducibility (precision), reliability, maximum yield, minimum total run-time and production cost.
It is also recognized that the present invention and its manufacturing process based on real-time spectral measurements, algorithmic processing, and closed-loop feedback control during the color filter deposition and formation processes uniquely enable extremely high accuracy and precision to be achieved, in clear distinction to the open-loop color processes of Prior Art. It is an object of the present invention to employ the versatility and flexibility of real-time metrology-based feedback control loops to correct, adjust, compensate, or find related applicability to finely tune the optical-train of the semiconductor color imager for optimum spectral response, maximum image light collection efficiency, and superior image resolution, dynamic range, and spectral signal contrast.
Another object of the present invention is to obviate topographical step variations and non-planar problems encountered with conventional Prior Art formation sequences. Prior Art is well known to have step-height or steric effect variations between R/G/B layers and results in departures from designer""s specifications in transmittance balance.
A further object of the present invention is to enable controlled adjustments during the coating sequence to obtain an increased range of specified R/G/B gain-ratio.
Avoidance of the specific color pixel lifting problem is a still further object of the present invention.
It is another principal object of the present invention to enable combinations and permutations of the color process variables, process sequence order, and, associated admixtures with the fabrication of microlens arrays, planarizing layers, spacer layers, and extensions to alternative classes or compositions of color filters beyond the dye or pigment absorption type used to illustrate the present invention.
In the present invention, the precedence flow-chart of the fabrication sequence creates the color filter array in the order B/R/G or B/G/R instead of the conventional G/R/B. It is an object of the present invention to enable dye or pigment concentration loading as an adjunct to the primary object of thickness adjustment control.
To practice the method of the present invention, conventional microelectronic fabrication techniques using photolithographic materials, masks and etch tools are employed: in succession the array of pn-junction photodiodes is patterned with impurity dopants diffused or ion-implanted, electrically isolated, and planarized over. A first pattern of precursor microlens structures, comprised of single or multiple coplanar lens elements, is formed, etched, and thermally reflowed.
The microlens array is subsequently planarized by a successive deposition step, and, if desired, a second pattern of precursor microlens structures, again comprised of single or multiple coplanar lens elements, is exposed, developed, and anisotropically etched to form a vertical aligned compound lens with the microlens elements of the first plane-array. As many planarizations and microlens array planes as desired may successively be formed by repetition of the previously described fabrication sequence. Following the completion of the microlens plane-array formation above the photodetector array, typically three more layers are built up additively with primary green, red, blue color filters formed by the addition of suitable dyes or pigments appropriate to the desired spectral transmissivity to be associated with specified photodetector coordinate addresses in the imager matrix. In the present invention the fabrication sequence orders the color filter formation as blue, red, green in accord with the specified several objects of the present invention.